RF cavities, which are used as resonators for radiofrequency electromagnetic fields, are typically hollow bodies having an electrically conductive outer wall. An RF generator arranged outside the cavity generates electromagnetic radiation with a high frequency, which is coupled through an opening in the outer wall of the cavity into the interior of the cavity. The alternating electromagnetic fields generated by the generator induce, inter alia, alternating currents which propagate along current paths present on the inner side of the outer wall. Depending on the generator frequency and the waveguide properties of the cavity, various resonant modes may be set up inside the RF cavity. Owing to the arrangement of the RF generator outside the cavity, however, currents are also induced on the outer side of the outer wall, by which the power coupled into the cavity and therefore the efficiency of the cavity may be reduced.